A carriage and mast assembly is commonly used on vehicles such as fork lift trucks to pick up and deliver loads from one area to another. The mast usually has a pair of movable inner uprights which can be raised vertically in relation to a pair of fixed outer uprights. In a carriage and mast assembly which has "free lift", the carriage is carried on the inner uprights and can be raised or lifted vertically without extending the inner uprights. Furthermore, after the carriage is fully lifted, the inner uprights and carriage can be raised in unison along the fixed outer uprights. The advantages of having these relative movements are well-known and need not be discussed further.
Free lift of the carriage is accomplished with prior carriage and mast assemblies by using lift cylinders and lift chains. For example, an assembly will include at least one lift cylinder having an outer jacket and a piston rod that can be extended from the jacket. At least one lift chain has one end coupled to the carriage and extends about a sheave connected to the piston rod, with the other end being fixed, for example, to the outer jacket. This lift chain provides an indirect coupling between the lift cylinder and carriage. When the piston rod is extended, the lift chain will move around the sheave and the carriage will be lifted with respect to the inner uprights to produce the free lift.
A disadvantage with prior free lift mechanisms is that the lift chains will wear over a period of use. This means that chain failure can occur and/or that the chains must be repaired or replaced. Also, the weight and cost of the carriage and mast assembly are undesirably increased due to the need for the lift chains, together with the mounting components such as the sheaves. Furthermore, during the lifting of the carriage or during movement of the lift truck, the lift chains can flap about, thereby marring or possibly damaging the lift cylinders.
Another disadvantage is that operator visibility is impaired with prior free lift mechanisms. Typically, the free lift cylinders and lift chains are disposed in the space between the inner uprights. The use of the lift chains, and/or this placement of the cylinders and chains, limits operator visibility through the mast.
Yet another disadvantage with prior free lift mechanisms is that the free lift cylinders are exposed to damage from objects near the floor. For example, in the lowered position, the carriage will be near the floor with the piston rod retracted into the cylinder jacket and the latter also located near the floor. The carriage, which is in front of the cylinder, can protect the cylinder from such objects. However, when the carriage is raised for free lift, the cylinder jacket is still near the floor and, thereby, exposed to damage from these objects. Furthermore, the carriage is normally loaded in the raised position, with the rod being extended from the jacket. This can cause the rod to bend.
The present invention is directed to overcoming one or more of the problems as set forth above.